There is a growing need for higher octane in the refinery gasoline pool, particularly since the phase-out of lead additives for gasoline both in the U.S. and abroad. Decreases in octane sensitivity, i.e., the difference between research and motor octane, are especially desirable. Increased alkylate and potential alkylate are also needed from today's gasoline manufacturing processes. Some C.sub.3 and C.sub.4 olefins are useful by-products of such a manufacturing process; increases in these olefins are desired. These light olefins are used to make ethers and/or alcohols.
Most options available to FCC operators have limited potential. Use of shape-selective cracking additives, or large-pore cracking catalyst containing such additives, appeared to have only limited potential to increase yields of light olefins.
Pyrolysis units or thermal crackers produce large amounts of olefins, but little gasoline. A high severity, shape-selective cracking process is also available. However, like the closely related pyrolysis process, the high severity process makes large amounts of olefins and relatively small yields of highly aromatic, low octane gasoline.
In efforts to solve these problems, a number of processes have been developed. For example, U.S. Pat. No. 3,758,403 teaches the benefits of adding ZSM-5 to conventional large-pore cracking catalyst formulations. Example 2 of the patent uses a catalyst consisting of 5 wt. % ZSM-5, 10 wt. % REY, and 85% clay. With a gas oil feedstock, the catalyst produced 11.42 vol. % propylene, and a total yield of alkylate and C.sub.5.sup.+ gasoline of 89.1 vol. %. Example 3 of the patent uses a catalyst consisting of 10 wt. % ZSM-5, 10 wt. % REY, and 80% clay. Although the ZSM-5 content doubled, propylene yields increased from 11.4 vol. % to only 13.6 vol. %. The total yield of alkylate and gasoline declined slightly, from 89.1 vol. % to 88.6 vol. %.
U.S. Pat. No. 3,847,793 teaches a slightly different approach. The ZSM-5, which could be in the same particle with the large-pore zeolite, or in a separate additive, is used to convert olefins to aromatics. A riser reactor with an enlarged upper portion is used, along with injection of a coking fluid near the top of the riser, to deactivate the large-pore catalyst while leaving the ZSM-5 catalyst active. Gasoline boiling range material could be injected into the top of the riser for conversion. Table 2 of the patent shows that this approach reduced the mono-olefin content of an FCC gasoline from 14.0 wt. % to 2.9 wt. %. The discussion of Example 2 reports that ZSM-5 was effective for converting propylene to aromatics over a wide range of catalyst silica/alumina ratios.
Based on U.S. Pat. No. 3,847,793, large amounts of ZSM-5 should efficiently convert propylene into aromatics. This would reduce light olefin production, and perhaps exacerbate problems of producing gasoline without exceeding aromatics and/or benzene specifications.
Based on U.S. Pat. No. 3,758,403, use of large-pore cracking catalyst with large amounts of ZSM-5 additive gives only modest increase in light olefin production. A 100% increase in ZSM-5 content (from 5 wt. % ZSM-5 to 10 wt. % ZSM-5) increased the propylene yield less than 20%, and decreased slightly the potential gasoline yield (C.sub.5 + gasoline plus alkylate).
Because refiners must retain the ability to use the many types of commercially available large-pore cracking catalysts available today, the normal practice is to use additive catalysts, with 10 to 50 wt. %, more usually 10 to 25 wt. % ZSM-5 in an amorphous support, to their FCC units. Such additives have physical properties which allow them to circulate with the large-pore cracking catalyst.
U.S. Pat. No. 4,309,280 teaches adding very small amounts of powdered, neat ZSM-5 catalyst, characterized by a particle size below 5 microns. Adding as little as 0.25 wt. % ZSM-5 powder to the FCC catalyst inventory increased LPG production 50%. Small amounts of neat powder behaved much like larger amounts of ZSM-5 disposed in larger particles.
A way to add a modest amount of ZSM-5 to an FCC unit is disclosed in U.S. Pat. No. 4,994,424, incorporated herein by reference. ZSM-5 additive is added to the equilibrium catalyst in a programmed manner so an immediate boost in octane number, typically 1/2-2 octane number, is achieved.
U.S. Pat. No. 4,927,523, incorporated herein by reference, teaches a way to add large amounts of ZSM-5 to a unit without exceeding wet gas compressor limits. Large amounts are added and cracking severity is reduced in the FCC unit for several days.
Recent work on ZSM-5 additives has been directed at stabilizing the additives with phosphorus or making them more attrition resistant. Phosphorus stabilized ZSM-5 additive is believed to retain activity for a longer time. Phosphorus stabilization thus reduces the makeup rate of ZSM-5 additive required. U.S. Pat. No. 5,110,776 teaches a method for preparing FCC catalyst comprising modifying the zeolite, e.g., ZSM-5, with phosphorus. U.S. Pat. No. 5,126,298 teaches manufacture of an FCC catalyst comprising zeolite, e.g., ZSM-5, clay, and phosphorus. Phosphorus treatment has been used on faujasite-based cracking catalysts for metals passivation (see U.S. Pat. Nos. 4,970,183 and 4,430,199); reducing coke make (see U.S. Pat. Nos. 4,567,152; 4,584,091; and 5,082,815); increasing activity (see U.S. Patents 4,454,241 and 4,498,975); increasing gasoline selectivity (See U.S. Pat. No. 4,970,183); and increasing steam stability (see U.S. Pat. Nos. 4,765,884 and 4,873,211).
One concern regarding use of ZSM-5 additive, even with phosphorus stabilization, is that refiners fear dilution of the large-pore cracking catalyst by addition of large amounts of ZSM-5, e.g., over 2 or 3 wt. % ZSM-5 crystal, or use of more than 5 or 10 wt. % additive, will seriously impair conversion since ZSM-5 has difficulty cracking the heavier molecules in gas oil feeds. Most refiners operate with significantly smaller amounts of ZSM-5 than the upper limits recited above.
Another concern is how well the unit will respond when pushed to make even more olefins. The consensus is that small amounts of ZSM-5 additive make large amounts of olefins in an FCC unit operating at low severity, but the increase in yields of light olefins attributable to ZSM-5 declines as severity increases. As reported in Elia, M.F. et al.,"Effect of Operation Conditions on the Behaviour of ZSM-5 Addition to a RE-USY FCC Catalyst", Applied Catalysis, 73, 195-216, 202 (1991), working at low severity produces an increase in light olefinic compounds, mostly branched, in the C.sub.5 -C.sub.6 range. At the same time, an increase in light branched alkanes results and the aromatics and naphthenes contents are almost not affected. Elia et al. report that when the cracking occurs at higher temperatures, an increase in the C.sub.7 -C.sub.8 aromatics and naphthenes is observed, but a much smaller increase in the lighter compounds results.
The poor response to unusually large concentrations of ZSM-5 was reported in U.S. Pat. No. 3,758,403, while Elia et al. have shown the unfavorable response of ZSM-5 to high severity FCC operation.
In summary, most refiners operating cracking units would prefer more light olefins, e.g., propylene and butylene. Based on the teachings of U.S. Pat. No. 3,758,403, use of ever increasing amounts of ZSM-5 and large-pore zeolite in a common particle produces rapidly diminishing returns from the incremental amounts of ZSM-5. Based on the state of the art on the use of separate additives in the catalytic cracking process, use of large amounts of additive comprising ZSM-5 would also produce diminishing returns at high severity. Today most refiners tend to use more severe operation to increase conversion, and improve gasoline yield and octane.
Based on the pyrolysis work reported in U.S. Pat. No. 4,980,053, use of large amounts of separate ZSM-5 additive at high severity reduces both conversion and gasoline yield, and would produce a highly aromatic gasoline.
Accordingly, it is an object of the present invention to provide an improved cracking process using an improved additive catalyst.
It is a further object of the invention to provide for the use of an improved additive catalyst composition to impart an octane-enhancing property in the present catalytic cracking process, and to enhance production of light olefins, e.g., propylene.
It is a particular object of the invention to provide for the use of an improved additive catalyst composition in hydrocarbon cracking to result in product rich in high octane gasoline, alkylate, gasoline plus potential alkylate, and petrochemical grade lower olefins, e.g., propylene.